Gravity measurements and terrain corrections using a digital terrain modelin the NW Himalaya

Areas recently gravity surveyed in the NW Himalaya are characterized by hig h-elevation and high-amplitude topographic undulations. A new method of app lying combined Bouguer and terrain corrections using a digital terrain mode l is highly accurate and offers advantages over conventional techniques by saving efforts and being more flexible. Partitioning parameters for, statio n-dependent inner-zone compartments and station-independent outer zones can be optimally selected for the desired accuracy requirements. A digital ter rain database is used to obtain the outer-zone corrections. In the situatio n of the NW Himalaya surveys, a 1.2 km inner zone is divided into 112 compa rtments for each station and a digital terrain database containing nearly 1 6 000 data points for 30" x 30" compartments was applied using the computer program EFFECT.FOR, to compute combined Bouguer and terrain corrections fo r a 20 km range. The terrain corrections between 20 and 170 km were compute d using National Geophysical Data Centre (NGDC) 5' x 5' gridded global elev ation database. The magnitude of the terrain correction varies between 3 an d 50 mGal. The effects of the 20 km range terrain correction are more prono unced on short-medium wavelength anomalies. The Swarghat gravity high is fu rther enhanced while several high-frequency pseudo-anomalies disappear afte r applying the terrain corrections. The refined Bouguer anomaly Varies from -160 mGal at the southern end of the section, to -310 mGal at the northern end, suggesting a Moho depth variation from 45 to nearly 60 km. The steepn ess of the northward negative gravity gradient, typical for the Himalaya, i s considerably reduced after applying a terrain correction for the 170 km r ange. (C) 1998 Elsevier Science Ltd. All rights reserved.